Conventional microcomputers have at least one reset pin (reset port) to which a reset signal for microcomputer reset is designed to be input. Specifically, the microcomputer is in shutdown state while the reset signal is in an active level, and is reset (rebooted) when the reset signal is turned from the active level to an inactive level.
On the other hand, when microcomputers need not control targets allocated thereby, they are programmed to operate in sleep mode as an example of low power consumption mode with low current drain as compared with in normal operation mode. Such a microcomputer operable in sleep mode is disclosed in, for example, Japanese Unexamined Patent Publication No. H09-146913.
Electronic devices equipped with such a microcomputer include electronic control units (ECUs) for vehicles.
In ECUs consisting of a microcomputer capable of operating in low power consumption mode, when a failure occurs in the microcomputer, the microcomputer is reset. When the microcomputer need not control targets allocated thereby, the operation mode of the microcomputer is configured to be shifted into low power consumption mode.
Specifically, in an ECU installed in a vehicle and consisting of a microcomputer capable of operating in low power consumption mode, the ECU is equipped with a power supply circuit operative to step down a battery voltage fed from a battery installed in the vehicle to thereby generate an operating voltage. The power supply circuit is operative to supply the operating voltage to the microcomputer.
When detecting that the operating state of the microcomputer is in abnormal, the power supply circuit works to turn a reset signal with an inactive level being input to the microcomputer to an active level. The power supply circuit also works to output, to the microcomputer, a control necessary/unnecessary signal indicative of the necessity of control for target devices allocated by the microcomputer depending on, for example, the on/off state of an ignition switch installed in the vehicle.
When the output level of the reset signal being input to the microcomputer is shifted from the inactive level to the active level, the microcomputer is reset. When the output level of the control necessary/unnecessary signal is shifted from a level representing the necessity of control for the target devices to that representing the unnecessity of control therefor, the microcomputer is shifted from normal operation mode into low power consumption mode.
In such a microcomputer with the reset function and the low power consumption mode shifting function, it is necessary to provide, to the microcomputer, at least one mode-control pin (port) via which the external control necessary/unnecessary signal can be input thereto independently of the reset pin via which the external reset signal can be input to the microcomputer.
In microcomputers used to be installed in ECUs, the greater the number of pins (ports) of a microcomputer is, the greater a package of the microcomputer is in size. This may deteriorate the installability of microcomputers in ECUs. For this reason, there are needs of users to reduce the number of pins of microcomputers as much as possible.
In order to address the user's requirements, no low power consumption mode pins can be provided to microcomputers, but in this method, they can not be shifted from normal operation mode into low power consumption mode. This may increase the current drain in microcomputers of ECUs installed in a vehicle, which may cause the life of the battery installed in the vehicle to be reduced.